1. Field of the Invention
The present invention generally relates to an optical system for observation, such as a telescope or a binocular. In particular, the present invention relates to an image stabilizing optical system in which a decentering optical system capable of decentering from the optical axis of the optical system is disposed and which can compensate for waver of an image, due to vibration produced for reasons such as movement of hands, by decentering the decentering optical system so as to deflect the image when the optical system vibrates.
2. Description of Related Art
An observation optical system such as a binocular having a waver compensator has been disclosed in, for example, Japanese Laid-Open Patent Publication No. 55-98718. The waver compensator disclosed in this publication spatially stabilizes a prism suspended by gimbals in the optical system by using a gyro-motor so as to stabilize an image.
Other optical systems having waver compensators have been disclosed in, for example, Japanese Laid-Open Patent Publications 2-271316 and 7-43646. The optical system disclosed in the former publication stabilizes the image by moving an erecting optical system with a supporting mechanism known as a cardan shaft. On the other hand, the optical system disclosed in the latter publication deflects the optical axis by deforming a variable angle prism so as to stabilize the image.
An image stabilizing optical system having a waver compensator utilizing a decentering optical system has been put to practical use in a camera lens which serves as a photographing optical system.
When movement of an erecting optical system is used to provide the waver compensation, a complicated large-sized device such as a gyro-motor, gimbals or the cardan shaft is required, resulting in an increase in size of the optical instrument. Therefore, the optical instrument incorporating such an erecting optical system cannot be adequately reduced in size.
In an image stabilizing optical system using the variable apex angle prism, when attempting to compensate for the waver of the image by the variable apex angle prism, lateral chromatic aberration by decentering increases. This degrades the image quality. However, it is very difficult to compensate for the chromatic aberration of magnification by decentering while simplifying the structure of the optical system.
On the other hand, a compensator for the chromatic aberration of magnification by decentering has been used. This compensator can satisfactorily correct the chromatic aberration of magnification by decentering during the waver compensation by appropriately selecting a material for the erecting prism or the variable apex angle prism. However, a practical material for the variable apex angle prism is extremely limited. This makes it difficult to correct the chromatic aberration of magnification by decentering in optical systems having various specifications when compensating for the waver and yet correct various types of chromatic aberration satisfactorily when the waver compensation is not performed.
Moreover, an image stabilizing optical system having a decentering optical system has been put to practical use in connection with a camera lens. However, the configuration of such an image stabilizing optical system for photography is complicated. Such a configuration is not suitable for applications to optical systems for observation such as telescopes and binoculars, which require relatively simple configurations. Therefore, there has been no proposal of an image stabilizing optical system for observation systems, such as a telescope or a binocular, which includes a waver compensator utilizing the decentering optical system and which has simple configuration and high optical performance.
It is an object of the present invention to provide an image stabilizing optical system for observation, such as a telescope or a binocular, in which a decentering optical system which can be shifted or tilted in a direction substantially perpendicular to an optical axis is disposed so as to deflect an image. Waver of an image due to the movement of hands or the vibration in a car as it is driven can be compensated for with a simple system configuration and high optical performance.
In order to achieve the above object, according to an aspect of the present invention, an image stabilizing optical system includes, in order from an object-side, an objective lens system including a first lens group having positive refractive power, a second lens group having negative refractive power, and an erecting prism system. An eyepiece lens system is used for magnifying an erected image formed on an eye-side of the erecting prism system by the objective lens system so as to allow the magnified erected image to be observed. The eyepiece lens system has a positive refractive power as a whole. In the image stabilizing optical system, the first lens group includes at least one positive lens and one negative lens. The second lens group is moved in a direction substantially perpendicular to an optical axis, thereby deviating the image so as to stabilize the image.
In the present invention, the second lens group is moved in the direction substantially perpendicular to the optical axis when vibration occurs due to movement of the hands or the like, so as to produce deviation of the image in a direction which cancels out the deviation of the image due to the vibration. A stabilized image is thereby obtained. In addition, the erecting prism system is disposed on the eye side of the second lens group in the present invention. This allows the erected image to be formed on the eye-side of the erecting prism system. Moreover, in the present invention, the eyepiece lens system for magnifying the erected image so that the magnified erected image can be observed is disposed at a position on the eye-side of the erecting prism system so that the optical system is useable as an afocal optical system such as the binocular optical system.